Conventionally, an intake manifold is known to include a surge tank which accepts intake-air mainstream mainly containing new air and plural branch passages branched from the surge tank to introduce the intake-air mainstream to an internal combustion engine one by one.
Furthermore, an intake manifold is known, which introduces intake-air substream containing components other than new air to an internal combustion engine, together with intake-air mainstream. For example, exhaust gas recirculated to the engine (EGR gas) or blowby gas (PCV gas) obtained by ventilating a crankcase of the engine is included in the intake-air substream, which is made to join the intake-air mainstream. Hereafter, gas to be introduced into a cylinder by being contained in the intake-air substream such as EGR gas or PCV gas may be referred as target gas.
By the way, for such an intake manifold, various measurements are considered in order to restrict the ratio of the target gas from varying between cylinders, as to components of the intake air drawn into each cylinder.
For example, an intake manifold of Patent document 1 introduces intake-air substream including EGR gas as the target gas together with the intake-air mainstream, to the engine. Moreover, in the intake manifold of Patent document 1, a surge tank has the same number of ports as the branch passages, and the ports oppose to respective upstream openings of the branch passages in the surge tank. According to the intake manifold of Patent document 1, the opening area of the port is made to be smaller as departing from a feed port of the intake-air mainstream in the surge tank.
The inflow amount of the intake-air mainstream becomes smaller as the branch passage is departing from the feed port, thereby the inflow amount of the intake-air substream can be reduced as the branch passage is located more distant from the feed port. Thus, the ratio of the target gas can be equalized between the cylinders.
Moreover, an intake manifold of Patent document 2 introduces intake-air substream including PCV gas as the target gas together with the intake-air mainstream to the engine, and includes a sub-stream passage connected to each of the branch passages so as to introduce the intake-air substream to each of the branch passages. The sub-stream passage is prepared so that a pressure loss of the intake-air substream becomes equal among all of the branch passages, from a feed port of the target gas to the connection port of the branch passage.
Thereby, the ratio of the target gas can be equalized between the cylinders.
However, according to the intake manifold of Patent document 1, in a case where intake air is introduced through a specific branch passage, the intake-air substream is introduced not only from a port opposing the specific branch passage but also from the other ports. For this reason, the ratio of the target gas does not necessarily become equal between the cylinders.
In contrast, according to the intake manifold of Patent document 2, in a case where intake air is introduced through a specific branch passage, the target gas is introduced only through a fixed passage from the feed port to the connection port of the specific branch passage, so it can be seen that the target gas will be distributed uniformly.
However, in the case where intake air is introduced through the specific branch passage in the intake manifold of Patent document 2, the intake-air substream includes a portion of intake air which remains in the other branch passages by drawing the remaining intake air through the sub-stream passage, in addition to a portion introduced from the feed port of the target gas. That is, the sub-stream passage will function also as a passage which communicates the branch passages with each other. Due to the function, in the case where intake air is introduced through the specific branch passage, a portion of the remaining intake air remaining in the other branch passage is drawn through the sub-stream passage. For this reason, the ratio of the target gas will vary between the cylinders according to the remaining amount of the target gas in the remaining intake air remaining in the other branch passage.